Equilibrium constants for the simultaneous liquid-phase syntheses of methyl tert-butyl ether (MTBE) and tert-butyl alcohol (TBA) were determined experimentally at 1.6 MPa in the temperature range 313-353 K using initial water/alcohol molar ratios ranging from 0 to 0.7. Experiments were performed in a batch reactor, the initial molar ratio of alcohol/isobutylene was varied from 0.5 to 2. To reach equilibrium, the macroporous sulfonic acid resin CT-275 was used as the catalyst. TBA synthesis reached chemical equilibrium faster than MTBE synthesis. The thermodynamic equilibrium constants and the enthalpy, free energy, and entropy changes of both reactions are reported as functions of temperature. Both systems behave nonideally, and the equilibrium constants were found to agree with those determined separately and reported in the literature. At 298.16 K, the standard molar enthalpy. changes are -33.9 +/- 4.6 kJ/mol for MTBE synthesis and -38.4 +/- 3.4 kJ/mol for TBA synthesis, and the results are compared with literature data.